Perimetry measures the central and peripheral visual fields to detect vision loss. While the subject fixates on a central target, test stimuli are presented at different locations in the field of view. The subject indicates if he or she sees the test stimulus. In static perimetry, a first stimulus is presented at a chosen location; if the subject does not see the stimulus, the size or intensity of the test object is increased in later presentations at the same location until the subject sees it. If the initial stimulus is seen, then stimulus size or intensity is decreased in subsequent presentations at the same location until not seen. This is repeated at a series of locations to determine the sensitivity of the visual field at each. In kinetic perimetry, the same test stimulus is moved from a peripheral area toward the center until the subject sees it. This is done from multiple directions, determining a boundary within which the subject can see the stimulus and outside of which the subject cannot see it. This can be repeated with multiple test stimuli.
Maintaining steady fixation on the central target is vital to accurate mapping of the visual field and for comparing follow-up tests over time. However, the full test procedure is often lengthy, and subjects may experience fatigue during the testing, making it difficult to maintain central fixation. Moreover, as the test stimulus is moved, a subject's natural inclination may be to move his or her gaze toward the stimulus rather than maintaining central fixation. Information regarding the subject's fixation during the test may be incorporated into the test results. The commercially available HFA II-i (Carl Zeiss Meditec, Inc. Dublin, Calif.) provides a graphical display of the gaze tracking output over time. The display indicates a magnitude of deviation but provides no information on where the subject is actually looking. In another method, the clinician may be given images of the patient's eye during each stimulus presentation, so the clinician can determine not only gaze but also whether there were other detractors, such as blinking (see U.S. Publication No. 2012/0274905 to Johansson, et al.).
Current protocols for perimetry typically require certain initial steps for calibrating gaze information. First, they may require an initial step that allows the system to determine whether the subject is properly centrally fixated during the presentation of test stimuli. During this step, the subject is generally instructed to fixate at a central location. In one type of gaze tracker used in the HFA II-i, a light source is used to create a reflection from the subject's cornea while the subject fixates at the central location. The spatial relationship between this corneal reflection and the location of the eye's pupil is then later used to determine whether, during the presentation of test stimuli, the subject appropriately maintains central fixation. Other gaze tracking designs may rely upon location of different ocular landmarks, but all require some sort of initial zero-point calibration in order to correct for normal anatomical variability from patient to patient. This initial calibration step takes anywhere from just a few seconds to tens of seconds and increases the total time required to complete a visual field examination. Thus, a method of gaze tracker calibration that can be accomplished in the normal course of test stimuli presentation during visual field testing, instead of before visual field testing, is desirable.
Some protocols may also include another calibration step that allows the system to more accurately determine the direction of the subject's gaze during the presentation of each stimulus, not just whether the gaze deviates from the central fixation point. In protocols seeking to have highly accurate gaze tracking, the subject may need to perform a lengthy calibration process in which an ocular reflection is tracked on the eye while the subject looks individually at each of a number of different reference locations. This determines a mapping function between a vector defined by the reflection and pupil location, versus the subject's gaze direction.
Because this calibration step is time-consuming for the subject to perform, many protocols do not acquire calibration data for each individual subject. Instead, they use a single mapping function to predict all subjects' gaze behavior. Relying on the same mapping function for all subjects, however, can be a source of error. The shape of subjects' eyes may vary from the reference eye, making the gaze estimates based on the reference eye's mapping function inaccurate.
Some gaze-tracking methods have been proposed to avoid this source of error while also avoiding time-consuming individual calibration steps. However, these methods require the use of multiple cameras and accurate camera calibration during manufacturing, adding significant cost to the system. Thus, a method that does not introduce errors through use of a single reference eye, while not adding to subject testing time, but also that does not require costly modifications to a diagnostic system, is highly desirable.